This invention relates to subharmonic double-balanced mixers and, in particular, to subharmonic double-balanced mixers which can be implemented by integrated circuit techniques.
There is currently a great amount of activity in realizing low cost Si based transceivers for the wireless communications market. All transceiver architectures use a mixer for the frequency translation from the RF (radio frequency) to the IF (intermediate frequency) and vice versa. A double-balanced Gilbert cell mixer is routinely used in most transceivers today where high LO (local oscillator) to RF isolation and high LO/RF suppression at the IF port is needed.
A mixer is a nonlinear device containing either diodes or transistors, the function of which is to combine signals of two different frequencies in such a way as to produce energy at other frequencies. The mixer is typically built using Silicon, GaAs, or InP diodes, or using Silicon-based transistor technology comprising but not limited to, BJT (bipolar Junction Transistors), MOS and CMOS transistors, BiCOMS (bipolar/CMOS), or using SiGe-based transistors (HBT, etc.), or using GaAs-based transistor technology (MESFET, HEMT, HBT, etc.) or using InP-based transistors technology (MESFET, HEMT, etc.).
In a typical down converter application within a heterodyne receiver, a mixer has two inputs and one output. One of the inputs is the modulated carrier RF or microwave signal at a frequency fRF, the other is a well controlled signal from a local oscillator at a fixed frequency, or a voltage controlled oscillator (VCO), at a frequency fLO. The result of down conversion is a signal at the difference frequency fRFxe2x88x92fLO or fLOxe2x88x92fRF, which is also called the intermediate frequency fIF. A filter is sometimes connected to the output of the mixer to allow only the desired IF frequency signal to be passed on for further processing. For example, for an RF frequency of 840 MHz and an LO frequency of 770 MHz, the IF frequency would be 70 MHz. Another example is an RF frequency of 1960 MHz and an LO frequency of 1820 MHz, resulting in an IF frequency of 140 MHz at the output of the mixer. FIG. 1 illustrates several other frequency selections for the RF, LO and IF signals.
The mixer circuit is also commonly used as an up converter. In this case, the low frequency (baseband or intermediate frequency) modulated signal is upconverted in the mixer using a local oscillator (fixed frequency or voltage controlled oscillator) to generate the modulated RF or microwave signal. The resulting signal of the up-conversion is at a frequency given by fLO+fIF, or fLOxe2x88x92fIF. A filter is sometimes used at the output of the up converter (mixer) to choose either one of these frequencies. In general, all the properties such as isolation between the IF, RF and LO ports, intermodulation, conversion loss, etc. of the up converter are closely related, if not identical, to the properties of the down converter mixer.
There are several kinds of receiver topologies which are commonly used today. The first one is commonly called the heterodyne receiver (or the superheterodyne receiver) which uses an intermediate frequency which is rather high, as is illustrated in FIG. 2. The IF signal can be selected to be anywhere, at 10 MHz, 40 MHz, 140 MHz, 220 MHz, 400 MHz, etc. and even 1 or 2 GHz in high frequency systems (RF of 5-20 GHz). The IF signal is filtered, amplified in a high gain IF amplifier chain, and then down converted again to a low frequency (typically called the second IF) for demodulation and detection.
The other receiver topology is called a direct conversion receiver which uses virtually no IF signals, and down converts the RF signal directly into baseband, as illustrated in FIG. 3. In this case, for an RF signal with a center frequency of 1960 MHz and a bandwidth of 1 MHz, one would choose an LO at 1960 MHz, resulting in an IF signal output of 0-1 MHz. The direct conversion receiver results in a much easier system layout, and also saves on using expensive IF filters, and the resulting IF electronics. The bandwidth of the baseband signal is typically given by the type of modulation used, and can be 0-200 KHz, 0-1 MHz, or even 0-5 MHz. There are also other kinds of receivers, such as the low-IF heterodyne receiver, which are less commonly used.
The direct conversion topology is much less expensive to build than the standard heterodyne receiver with its IF electronics, and will miniaturize the front-end electronics even further than today""s state-of-the-art. It is expected to be used in the new wireless local loop transceivers at 2.4 GHz, 3.5 GHz, 4.9 GHz, 5.8 GHz, etc., in low-cost cordless telephones at 900-1100 MHz, and in the new digital cellular telephones (1700-2100 MHz).
One must be very careful in mixer designs to ensure that no LO power and no RF power leak into the IF port. The reason is that the IF port is typically followed by a high gain amplifier and any LO leakage can saturate this IF amplifier. Also, any RF or LO leakage can introduce intermodulation products in the IF amplifier and limit the sensitivity of the receiver. This is typically done using a filter at the IF port which is selected to pass the IF signal, and to greatly attenuate the RF and LO signals. This embodiment is common in single and balanced mixer designs, but the filter occupies a lot of space on the RF integrated circuit. In reality, and for RFIC applications, single and balanced mixer designs are rarely used because of the existence of the Gilbert-Cell double-balanced mixer as described hereinbelow. However, balanced mixers built with an integral IF filter are typically used at microwave frequencies (5 GHz, 10 GHz, etc.) since they offer acceptable performance without taking a lot of space on the MMIC wafer.
The double-balanced mixer topology results in no RF and LO leakage at the IF port, and therefore no IF filter is used with this topology. The xe2x80x9cdouble-balanced mixerxe2x80x9d is the most commonly used topology in integrated circuit mixers. A double-balanced mixer is essential in RF-IC mixers since it mitigates the use of complicated and expensive on-chip (or off-chip) filters to remove the LO and RF signals at the output IF port. Double-balanced mixers are typically built using a xe2x80x9cGilbert-Cellxe2x80x9d topology in RFIC transistor circuits, as is illustrated in FIG. 4, or using a transformer-coupled diode-ring circuit. In RFIC applications which use a Gilbert-Cell topology, the double-balanced mixer requires that the LO and the RF frequencies be relatively close to each other (RF at 1960 MHz, LO at 1820 MHz and with an IF of 140 MHz).
A subharmonic double-balanced mixer has the same function as a regular double-balanced mixer, and still offers high isolation between the RF and IF ports, LO and IF ports and RF and LO ports, but uses an LO frequency which is approximately half that of the RF signal, as illustrated in FIG. 5. The IF signal frequency is therefore the difference frequency between RF signal frequency and 2 times the LO signal frequency (fIF=fRFxe2x88x922fLO, or fIF=2fLOxe2x88x92fIF). In the case of heterodyne architectures with an RF signal at 1900 MHz (or 2400 MHz), rather than use an 1830 MHz LO (or a 2330 MHz LO), one could use a 915 MHz LO (or a 1165 MHz LO) to achieve an IF of 70 MHz. The use of a lower frequency local oscillator frequency simplifies the LO design, and most importantly, since the LO is at a lower frequency, one can integrate the LO on the RFIC chip and still satisfy the system phase-noise requirements. One can immediately see the need of this mixer for the new PCS bands and wireless local loop bands at 1900 MHz and 2400 MHz.
The subharmonic double-balanced mixer is also very beneficial to dual-band cellular telephones since only one LO signal is needed to cover both the 800 MHz band and the 1900 MHz band, and an example of such a system is illustrated in FIG. 6. This will save the designer a lot of space and cost in the RF front-end, will greatly simplify the system architectures, and will eliminate the cross-talk problems commonly available in dual-band systems which use two different LO frequencies.
The direct conversion topology mitigates the use of IF processing thereby resulting in a low cost receiver. However, direct conversion transceivers employing the standard double-balanced mixer have suffered from a serious technical problem, which is the feedthrough (leakage) of the LO signal to the RF port, its reflection back into the standard double-balanced mixer, its eventual self-mixing in the standard double-balanced mixer and the generation of a random DC offset at the output IF port. There are several leakage paths between the LO source and the RF port, some of which are shown in FIG. 7. This random DC offset signal is added to the IF signal, and can saturate the high-gain IF (baseband) amplifiers thereby limiting the receiver sensitivity. To date, there has not been a good solution to the LO self-mixing problem using double-balanced mixers.
The subharmonic double-balanced mixer further offers an excellent solution to the LO self-mixing problem as illustrated in FIG. 8. The reason is that if any LO leaks to the RF port and reflects back into the new double-balanced subharmonic mixer, it will mix with 2fLO and therefore will not generate any DC offset problems. To continue the example given above for the direct conversion receiver, for an RF signal of 1960 MHz with a bandwidth of 1 MHz, the LO is at half the RF frequency (flo=980 MHz), and the self-mixing of the leakage LO signal in the subharmonic double-balanced mixer generates a signal at the IF port which is again at the LO frequency (fif=f2loxe2x88x92flo=960 MHz). This output signal frequency is very far away from the required down converted IF (baseband) signal which is at 0-1 MHz, and can be easily filtered out using an on-chip capacitor, and therefore, does not saturate the high-gain baseband amplifiers.
The subharmonic double-balanced mixer also offers the key solution to the problem of LO self-mixing in direct conversion receivers and allows the construction of high performance direct conversion receivers using low cost RFIC techniques. It is very important that a subharmonic double-balanced mixer is used, and not just a subharmonic balanced mixer. The reason is that a subharmonic double-balanced mixer offers high isolation between all three ports (RF, LO and IF) without any external filters, and eliminates the problem of LO self-mixing.
The same concept for the subharmonic mixer operates also on the subharmonic double-balanced up-converter. In this case, the subharmonic double-balanced up converter will translate a modulated IF or baseband signal using a local oscillator signal to result in a modulated RF or microwave signal, with high isolation between the IF, LO and RF ports, and without resulting in any DC output at the RF port due to the LO self-mixing problem.
There have been few implementations of subharmonic mixers to date. The easiest implementation is to first double the LO frequency using a frequency xe2x80x9cdoublerxe2x80x9d, and then use the doubled LO frequency in a standard double-balanced mixer. This circuit was first used by Kimura et al., published in 1994 (Ref 2) and then by Meyer et al., published in 1997 (Ref 3). While this circuit may be applicable to low cost heterodyne receivers with non-stringent intermodulation requirements, it still does not solve the LO self mixing problem since the doubled LO signal can still leak to the antenna or the RF port and mix with itself in the standard double-balanced mixer, as illustrated in FIG. 9.
Another implementation was presented by Sheng et al., published in 1999 (Ref 4). In this case, the LO is divided into four signals, each 90 degrees in phase from each other (LO signals are at 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0) and these signals are fed to a three-level transistor Gilbert-cell type mixer, as illustrated in FIG. 10. This embodiment of the subharmonic double-balanced mixer results in a three-level transistor circuit and therefore requires a higher supply voltage than Gilbert cell mixers which are commonly built using a two-level transistor circuit.
The references cited in this application are more fully identified as follows:
1. B. Gilbert, xe2x80x9cFour-Quadrant Multiplier Circuitxe2x80x9d. U.S. Pat. No. 3,689,752;
2. K. Kimura and H. Asazawa, xe2x80x9cFrequency Mixer with a Frequency Doubler for Integrated Circuitsxe2x80x9d, IEEE J. SOLID-STATE CIRCUITS, Vol. 29, No. 9, pp. 1133-1137, September 1994;
3. R. G. Meyer, W. D. Mack and J. J. E. M. Hageraats, xe2x80x9cA 2.5-GHz BiCMOS Transceiver for Wireless LANsxe2x80x9d, IEEE J. SOLID-STATE CIRCUITS, Vol. 32, pp.2097-2104, December 1997;
4. L. Sheng, J. Jensen, L. Larson, xe2x80x9cA Si/SiGe HBT Sub-Harmonic Mixer/Downconverterxe2x80x9d, IEEE BCTM proceedings, pp. 71-74, September 1999; and
5. B. Gilbert. xe2x80x9cThe Multi-tanh Principle: A Tutorial Overviewxe2x80x9d, IEEE J. SOLID-STATE CIRCUITS, Vol. 33, No. 1, pp. 2-17, January 1998.
An object of the present invention is to provide a subharmonic double-balanced mixer which does not consume a significant amount of power and which works on standard, low voltage batteries. A two-level transistor circuit is preferably utilized in the mixer to yield a high performance. The mixer operates at very low voltages (around 2-2.7 V) and consumes a low amount of power.
Another object of the present invention is to provide a low cost subharmonic double-balanced mixer which is very small and which are compatible with Si, GaAs and InP RFICs and MMICs for wireless communication systems.
Yet another object of the present invention is to provide a subharmonic double-balanced mixer which has wide RF bandwidth (more than 100 MHz at 1900 MHz input) and a high rejection of both the LO and the RF power at the IF port.
Yet another object of the present invention is to provide a subharmonic double-balanced mixer which can solve the problem of LO self-mixing in direct conversion receivers.
In carrying out the above objects and other objects of the present invention, a non-linear device for combining a first signal having a first frequency and first, second, third and fourth drive signals each having a second frequency approximately one-half the first frequency to obtain an output signal having a third frequency is provided. The device includes a first circuit section which has an input port to receive the first signal. The first signal can either be single-ended or differential. The device also includes a second circuit section having first, second, third and fourth drive ports adapted to receive the first, second, third and fourth drive signals, respectively. The second drive signal is approximately 180xc2x0 out of phase with respect to the first drive signal and the fourth drive signal is approximately 180xc2x0 out of phase with respect to the third drive signal. The first drive signal is approximately 90xc2x0 out of phase with respect to the third drive signal and the second drive signal is approximately 90xc2x0 out of phase with respect to the fourth drive signal. In other words, the first and second drive signals form a first differential signal, the third and fourth drive signals form a second differential signal, and the first and second differential signals are driven in quadrature or are approximately 90xc2x0 out of phase with respect to each other. The first circuit section and the second circuit section are connected together to generate the output signal at the third frequency which depends on the first signal at the first frequency and the first, second, third and fourth drive signals at the second frequency.
The first signal is preferably an RF signal and the first circuit section is preferably an RF circuit section. The first, second, third and fourth drive signals are preferably LO signals driven at the phases mentioned above and the second circuit section is preferably an LO circuit section. The third frequency is either a sum of or a difference between the first and twice the second frequency depending on whether the device is used in upconversion or downconversion.
In one embodiment of the invention, the second circuit section includes first and second circuits. The first circuit is driven in a quadrature manner relative to the second circuit by the first and second drive signals, and the third and fourth drive signals, respectively. The first circuit includes a first pair of interconnected transistors which are driven by the first and second drive signals in a differential manner to generate a first current signal switching at twice the second frequency. The second circuit includes a second pair of interconnected transistors which are driven by the third and fourth drive signals in a differential manner to generate a second current signal which has a phase difference of approximately 180xc2x0 from the first current signal and is also switching at twice the second frequency. The first circuit section is connected to the second circuit section and includes the input port to receive the first signal at the first frequency and the output port to obtain the output signal at the third frequency. The double-balanced mixing between the first signal at the first frequency and the first and the second current signals at the twice the second frequency in the first circuit section generates the output signal at the third frequency which is a difference between (or a sum of) the first frequency and twice the second frequency when the circuit is used as a downconversion (or an upconversion) mixer, respectively.
In another embodiment of the invention, the second circuit section includes first and second pairs of circuits. The first pair of circuits are driven in a quadrature manner relative to the second pair of circuits by the first and second drive signals and the third and fourth drive signals, respectively. Each of the first pair of circuits includes a pair of interconnected transistors which are driven by the first and second drive signals in a differential manner. Each of the second pair of circuits includes a pair of interconnected transistors which are driven by the third and fourth drive signals in a differential manner. The first circuit section is connected to the second circuit section and includes the input port to receive the first signal at the first frequency. The first circuit section generates first and second current signals at the first frequency. The double-balanced mixing between the first and second current signals at the first frequency and twice the second frequency occurs in the second circuit section due to interconnections of the transistors in the second circuit section and generates the output signal at the third frequency which is a difference between (or a sum of) the first frequency and twice the second frequency when the circuit is used as a downconversion (or upconversion) mixer.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.